Lithium-ion (Li-ion), lithium metal, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds relatively having high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites at the anode that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. Specific cycling stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway.
Many attempts have been made to address the dendrite-related issues. However, despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. An urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries and other rechargeable batteries.
Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-220 Wh/kg, most typically 150-180 Wh/kg. These specific energy values are two to three times lower than what would be required for battery-powered electric vehicles to be widely accepted.
With the rapid development of hybrid (HEV) and plug-in hybrid electric vehicles (HEV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S8+16Li⇄8Li2S that lies near 2.2 V with respect to Li+/Lio. This electrochemical potential is Approximately ⅔ of that exhibited by conventional positive electrodes. However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Values can approach 2,500 Wh/kg or 2,800 Wh/l based on the cathode weight or cathode volume (not based on the total cell weight or volume), respectively, assuming complete reaction to Li2S. However, the current Li-sulfur products of industry leaders in sulfur cathode technology, have a maximum cell specific energy of 400 Wh/kg (based on the total cell weight).
In summary, despite its considerable advantages, the Li—S cell is plagued with several major technical problems that have hindered its widespread commercialization:    (1) Conventional lithium metal cells still have dendrite formation and related internal shorting issues;    (2) Sulfur or sulfur-containing organic compounds are highly insulating, both electrically and ionically. To enable a reversible electrochemical reaction at high current rates, the sulfur must maintain intimate contact with an electrically conductive additive. Various carbon-sulfur composites have been utilized for this purpose, but only with limited success owing to the limited scale of the contact area. Typical reported capacities are between 300 and 650 mAh/g (based on carbon-sulfur composite weight) at moderate rates.    (3) The cell tends to exhibit significant capacity degradation on repeated discharge-charge cycling. This is mainly due to the high solubility of the polysulfide anions formed as reaction intermediates during both discharge and charge processes in the polar organic solvents used in electrolytes. During cycling, the polysulfide anions can migrate through the separator to the Li negative electrode whereupon they are reduced to solid precipitates (Li2S2 and/or Li2S), causing active mass loss. In addition, the solid product that precipitates on the surface of the positive electrode during discharge becomes electrochemically irreversible, which also contributes to active mass loss.    (4) More generally speaking, a significant drawback with cells containing cathodes comprising elemental sulfur, organosulfur and carbon-sulfur materials relates to the dissolution and excessive out-diffusion of soluble sulfides, polysulfides, organo-sulfides, carbon-sulfides and/or carbon-polysulfides (hereinafter referred to as anionic reduction products) from the cathode into the rest of the cell. This phenomenon is commonly referred to as the Shuttle Effect. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact to active cell components, fouling of the anode surface giving rise to malfunction of the anode, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.
The description of prior art will be primarily based on the references listed below:                1. Choi, J.-W. et al. Rechargeable lithium/sulfur battery with suitable mixed liquid electrolytes. Electrochim. Acta 52, 2075-2082 (2007).        2. Shin, J. H. & Cairns, E. J. Characterization of N-methyl-N-butylpyrrolidinium bis(trifluoro-methanesulfonyl)imide-LiTFSI-tetra(ethylene glycol) dimethyl ether mixtures as a Li metal cell electrolyte. J. Electrochem. Soc. 155, A368-A373 (2008).        3. Yuan, L. X. et al. Improved dischargeability and reversibility of sulfur cathode in a novel ionic liquid electrolyte. Electrochem. Commun. 8, 610-614 (2006).        4. Ryu, H.-S. et al. Discharge behavior of lithium/sulfur cell with TEGDME based electrolyte at low temperature. J. Power Sources 163, 201-206 (2006).        5. Wang, J. et al. Sulfur-mesoporous carbon composites in conjunction with a novel ionic liquid electrolyte for lithium rechargeable batteries. Carbon 46, 229-235 (2008).        6. Chung, K.-I., Kim, W.-S. & Choi, Y.-K. Lithium phosphorous oxynitride as a passive layer for anodes in lithium secondary batteries. J. Electroanal. Chem. 566, 263-267 (2004).        7. Visco, S. J., Nimon, Y. S. & Katz, B. D. Ionically conductive composites for protection of active metal anodes. U.S. Pat. No. 7,282,296, Oct. 16 (2007).        8. Kobayashi, T. et al. All solid-state battery with sulfur electrode and thio-LISICON electrolyte. J. Power Sources 182, 621 (2008).        9. Xiulei Ji, Kyu Tae Lee, & Linda F. Nazar, “A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries,” Nature Materials 8, 500-506 (2009).        
In response to these challenges, new electrolytes [Ref. 1-5], protective films [Ref. 6-7] for the lithium anode, and solid electrolytes [Ref 8] have been developed. Some interesting cathode developments have been reported recently to contain lithium polysulfides; but, their performance still fall short of what is required for practical applications. For instance, Ji, et al [Ref. 9] reported that cathodes based on nanostructured sulfur/carbon materials could overcome the lithium polysulfide migration to a large degree, and exhibit stable, high, reversible capacities with good rate properties and cycling efficiency. However, the fabrication of the proposed highly ordered porous carbon structure requires a tedious and expensive template-assisted process. Furthermore, this porous carbon material does not accommodate a high loading of the cathode active material.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells containing elemental sulfur, organo-sulfur and carbon-sulfur cathode materials, or derivatives and combinations thereof, there remains a need for materials and cell designs that retard the out-diffusion of anionic reduction products, from the cathode compartments into other components in these cells, improve the utilization of electro-active cathode materials and the cell efficiencies, and provide rechargeable cells with high capacities over a large number of cycles.
Most significantly, lithium metal (including pure lithium, alloys of lithium with other metal elements, or lithium-containing compounds) still provides the highest anode specific capacity as compared to essentially all anode active materials (except pure silicon, but silicon has pulverization issues). Lithium metal would be an ideal anode material in a lithium-sulfur secondary battery if dendrite related issues could be addressed. In addition, there are several non-lithium anode active materials that exhibit high specific lithium-storing capacities (e.g., Si, Sn, SnO2, and Ge as an anode active material) in a lithium ion battery wherein lithium is inserted into the lattice sites of Si, Sn, SnO2, or Ge in a charged state. These have been largely ignored in the prior art Li—S cells.
Hence, an object of the present invention is to provide a rechargeable Li-metal or Li-ion battery that exhibits an exceptionally high specific energy or high energy density. One particular technical goal of the present invention is to provide a Li metal-sulfur or Li ion-sulfur cell with a cell specific energy greater than 500 Wh/Kg, preferably greater than 600 Wh/Kg, and more preferably greater than 800 Wh/Kg.
Another object of the present invention is to provide a lithium metal or Li-ion cell that exhibits a high specific capacity (higher than 1,200 mAh/g based on the sulfur weight or higher than 1,000 mAh/g based on the cathode composite weight, including sulfur, conducting additive and conductive substrate, and binder weights combined, but excluding the weight of cathode current collector). The specific capacity is preferably higher than 1,400 mAh/g based on the sulfur weight alone or higher than 1,200 mAh/g based on the cathode composite weight. This must be accompanied by a high specific energy, good resistance to dendrite formation, and a long and stable cycle life.
It may be noted that in most of the open literature reports (scientific papers), scientists choose to express the cathode specific capacity based on the sulfur or lithium polysulfide weight alone (not total cathode composite weight), but unfortunately a large proportion of non-active materials (those not capable of storing lithium, such as conductive additive and binder) is typically used in their Li—S cells. For practical use purposes, it is more meaningful to use the cathode composite weight-based capacity value.
A specific object of the present invention is to provide a rechargeable lithium-sulfur cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional Li—S cells: (a) dendrite formation (internal shorting); (b) extremely low electric and ionic conductivities of sulfur, requiring large proportion (typically 30-55%) of non-active conductive fillers and having significant proportion of non-reachable or non-usable sulfur or lithium polysulfides), (c) low active material utilization rate (a significant portion of the cathode active material not accessible or not participating in lithium storage), and (d) dissolution of lithium polysulfide in electrolyte and migration of dissolved lithium polysulfides from the cathode to the anode (which irreversibly react with lithium at the anode), resulting in active material loss and capacity decay (the shuttle effect).
Another specific object of the present invention is to provide a Li—S secondary cell wherein the cathode initially comprises an integrated structure of exfoliated graphite worms with lithium sulfides (Li2Sx, x=1-8, representing partially or fully oxidized state of sulfur) dispersed in the pores between thin graphite flakes and wherein lithium polysulfides are in fine powder form or a thin coating bonded to or coated on thin graphite flake surfaces of the graphite worms. During the first charge operation, lithium polysulfides are reduced to sulfur, releasing and sending Li+ ions back to the anode. This makes it possible not to handle lithium foil in a real battery cell production environment. This otherwise has been a challenging task, requiring an ultra-dry facility (<a few % of moisture).
Alternatively, one could use a high-capacity anode active material (such as Si) that is prelithiated to allow the use of sulfur directly in the cathode. The sulfur active material is also preferably in fine powder form or a thin coating bonded to or coated on thin graphite flake surfaces.
During subsequent charge and discharge operations, lithium polysulfides are essentially retained in the pores constituted by the interconnected thin graphite flakes at the cathode, permitting only Li+ ions to diffuse back and forth between the anode and the cathode. Surprisingly, this has been effective in reducing or eliminating the shuttle effect.
Another object of the present invention is to provide a simple (not too complex), cost-effective, and easy-to-implement approach to preventing potential Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal-sulfur batteries featuring the aforementioned meso-porous cathode and a nano-structured anode. Specifically, a nano-structured composition is used as an anode substrate of a Li metal-sulfur cell that enables uniform deposition of returning lithium ions during recharge, reducing or eliminating the formation of lithium dendrites. This nano-structured anode is assembled together with a meso-porous exfoliated graphite worm cathode.
Yet another object of the present invention is to provide a lithium-ion cell wherein the anode comprises a nano-structured composition (integrated structure of conductive nano-filaments and high-capacity anode active material such as particles, nano-wires, or thin coating of Si) and the cathode comprises a meso-porous exfoliated graphite worm structure to support sulfur. The integrated structure, being highly conductive, can also function as a current collector, obviating the need to have a separate current collector at either the anode or the cathode side. Such a configuration can significantly reduce the overhead weights of a cell, thereby affording an ultra-high specific capacity and specific energy.